Nerve growth factor (NGF) is an important growth regulatory polypeptide which promotes the survival and differentiation of immature neuroblasts in the developing nervous system. Unlike most growth hormones which stimulate cell replication, NGF inhibits cell proliferation and induces the maturation of cells bearing specific NGF receptors. Thus, the NGF-NGF receptor interaction serves as an important model system to use in investigating the normal mechanisms which regulate cell growth and differentiation. Furthermore, since the effects of NGF extend even to neoplastic cells bearing functional receptors, NGF is of potential clinical significance as a growth inhibitor of neoplasias of neural crest origin. Although NGF was the first growth factor to be identified, its mechanism of action is not well understood, except that its actions are initiated by binding to a high affinity cell surface receptor. This proposal is directed towards defining the role of the NGF receptor in initiating cellular responses to NGF, using the recently cloned gene that encodes the human NGF receptor. A combined molecular biological and biochemical approach will be used to determine the structure of the functionally active high affinity receptor. Specifically we plan to: (a) transfect the human NGF receptor clone into cells derived from the neural crest to determine the cellular characteristics which are responsible for responsiveness to NGF; (b) define the molecular basis of the high and low affinity forms of the receptor by membrane fusion experiments using PEG-mediated fusion; (c) define the domains of the receptor responsible for high affinity ligand binding, internalization and interactions with cell membrane proteins; (d) biochemically characterize the high affinity receptor complex using affinity chromatography and (e) use gene transfer to identify other gene products in the cell membrane which are required for functionally active high affinity receptor.